The invention relates to couplings for non-ferrous pipe (or tubing) of plastic or fiberglass, for example.
As referred to in prior U.S. Pat. No. 3,784,239, the use of non-ferrous pipe has been hindered by problems in effectively joining one pipe section to another. One form of coupling suitable for this purpose is described in the patent and comprises complementary threaded male and female coupling elements bonded to respective pipe ends of a pair of pipe sections so that the coupling elements may be threadably engaged and disengaged to couple and uncouple the sections. The male coupling element (sleeve) has a tapered threaded terminal portion adapted to be received in a corresponding tapered portion of the female coupling element, and the female element, which may be in the form of a filament-wound collar, carries an O-ring seal which engages around the tapered terminal portion of the male element. The patent also describes a method of making the female elements (collars) by a filament winding technique.
Couplings as described in the patent are generally suitable for use in diverse applications. However, sealing between the pipe sections is dependent on the coupling elements being threaded together sufficiently tightly so that the respective tapered portions suitably compress the O-ring therebetween. Thus, the degree of tightening is critical, and undertightening may lead to leakage while overtightening may make separation of the coupling difficult. It has also been found that in couplings using filament-wound female coupling elements, high working pressures within the coupling may produce fine cracking or crazing on the interior surface of the female element resulting in leakage of fluid therethrough.
The present invention provides a coupling in which sealing is substantially independent of the degree of tightening between the respective coupling elements, and which is not subject to leakage of fluid through the female coupling element as in the prior couplings.